Bone cement mixing and delivery systems are well known for mixing liquid and powder components of bone cement and delivering the prepared bone cement to an anatomical site during various surgical procedures. Bone cement is particularly useful in orthopedic procedures in which a prosthetic device is fixed to a bone or joint structure to improve the strength, rigidity, and movement of the structure. In a total hip arthroplasty (THA) procedure, in which a hip joint is replaced with a prosthetic device, bone cement is used to fix the prosthetic device in place in a medullary canal of a femur.
Typically, the bone cement is prepared in a mixing cartridge. The mixing cartridge includes a cylinder having proximal and distal ends with a mixing chamber defined between the ends. The mixing cartridge further includes a cap covering the proximal end of the cylinder and a piston disposed in the distal end of the cylinder such that the mixing chamber is further defined between the cap and the piston. The piston may be releasably secured in a locked position in the cylinder by a cotter pin. The cap supports a mixing device, i.e., a mixing shaft and blade, for mixing the liquid and powder components of the bone cement in the mixing chamber.
Once the bone cement is mixed, the mixing cartridge is prepared for inserting into a delivery gun to discharge the bone cement. This may include disengaging the mixing shaft and coupling a nozzle to the cap to provide a discharge point for the bone cement. At the same time, the piston is released from the locked position in the distal end of the cylinder by pulling the cotter pin. This allows the piston to be driven by the delivery gun through the mixing chamber to discharge the bone cement from the nozzle. An alternative solution for securing and releasing the piston is shown in U.S. Pat. No. 5,328,262 to Lidgren et al.
In Lidgren et al., the piston is releasably secured in the locked position in the distal end of the cylinder by a gripping portion in the form of a flange, which extends along only a portion of an inner periphery of the cylinder. The piston in Lidgren et al. has a corresponding gripping portion in the form of an outwardly directed lip that protrudes behind the flange. The lip defines a groove with an outer surface of the piston to receive the flange. To release the piston from the locked position, the flange is rotated through the groove until the flange has been rotated past the lip. Lidgren et al. discloses a base that is used to secure the piston from rotation while a user rotates the cylinder relative to the piston to release the piston from the locked position. This method of releasing the piston from the locked position, much like pulling the cotter pin, requires additional manipulation by a user.
Once the piston is released from the locked position, the mixing cartridge is inserted into the delivery gun. A typical delivery gun includes a ram disk that engages the piston and drives the piston through the mixing chamber to discharge the bone cement from the nozzle. The delivery gun includes a cradle for supporting the mixing cartridge and a casing for supporting a drive rod that engages the ram disk and advances the ram disk to drive the piston. The drive rod includes a plurality of teeth and a pawl member engages the teeth to advance the drive rod. A trigger supports the pawl member and the casing rotatably supports the trigger. Actuation of the trigger relative to the casing urges the pawl member against the teeth to advance the drive rod.
An example of such a delivery gun is illustrated in U.S. Pat. No. 5,431,654 to Nic. In the '654 patent to Nic, two pawl members are used to independently advance the drive rod and the ram disk. The pawl members provide high speed/low force and low speed/high force advancement of the drive rod. A switch is used to select between the speeds. When high speed is selected, both pawl members engage the drive rod, while only the high-speed pawl member actually advances the drive rod. When low 1′ speed is selected, the high-speed pawl member is isolated from the teeth such that only the low speed pawl member engages the teeth to advance the drive rod. However, in Nic, the trigger directly supports each of the pawl members which results in a low mechanical advantage to advance the drive rod and ram disk.